The present invention relates to a device and a method for determining the frequency and/or amplitude of a vibrating structure, particularly for the measurement of accelerations or angular rates.
Vibrating structures have various engineering applications, For example, they are used as acceleration and/or angular rate sensors, where movement quantities can be determined from the vibration characteristics.
The U.S. Pat. No. document 4,598,585 describes an angular rate sensor where a mass is supported around two axes (x-axis, y-axis) that are perpendicular to one another in a vibrating manner. To determine the angular rate around the z-axis that runs vertically to the x- and y-axis, the element is excited to a periodic vibration around the y-axis. The Coriolis forces during the rotation cause an additional vibration of the element around the x-axis, whereby the amplitude is a measure for the angular rate. Such an angular rate sensor with dual cardanic suspension is shown in FIG. 3.
Additionally, acceleration sensors are known that carry out an oscillating movement, whereby the frequency is a measure for acceleration acting in a certain direction.
However, the known vibrating structures have the disadvantage that no distinction can be made between a change in vibration amplitude and a parallel shift of the vibrating element. For example, distance changes of the suspension of the moveable element may occur that falsify the measured quantity. When sensing the position or the deflection of the vibrating element with a capacitor, the smallest changes in the distance of the capacitor plates can overlay the actual vibration amplitude such that exact measurement results are no longer possible. This applies particularly in cases where the interference or the parallel shift is in the same frequency range as the excited vibration.
It is, therefore, the objective of the present invention to create a device for the determination of the frequency and/or amplitude of a vibrating structure and to provide a method for measuring the vibration amplitude with a high degree of accuracy, whereby interferences, e.g., due to a parallel shift or a distortion of the structure can be suppressed effectively.
The device according to the invention for the determination of the frequency and/or the amplitude of a vibrating structure is particularly suited to measure the acceleration or angular velocity and has a movable element that can be excited in vibration, a pair of position sensors to determine the deflection of the movable element, that are arranged such that during a half-wave of the vibration their measurements exceed and/or fall short of each other, a circuit for comparing the measurements of the two position sensors for determining from the measurements a threshold value for the half-wave of the vibration, and a device for determining the duration during which the measurement of one of the two position sensors exceeds and/or falls short of the threshold value.
The special arrangement of the position sensors and the comparison of the measurements with threshold value determination in one half-wave of the vibration has the effect that vibration amplitude is determined accurately even with a measurement interference due to a parallel shift of the vibrating element. Errors in determining the amplitude are avoided.
Advantageously, the device comprises another pair of distance sensors to determine a second threshold value for the second half-wave of the vibration. These distance sensors are preferably arranged such that the threshold values in the positive and in the negative half-wave have the same value. Especially in a mechanical structure that vibrates around a defined axis, an acceleration that acts perpendicular to the vibration axis A will no longer result in errors in the amplitude measurement.
Advantageously, the position sensors are capacitors, which results in a cost-effective design. However, they may also be made of optical elements or similar sensors known to the professional for position or distance measurements.
Preferably, one element of a pair of position sensors is arranged higher such that the threshold is set at a defined deflection of the movable element. The threshold values can, for example be activated through mechanical stages, or step-like arrangement of electrodes at a vibration element or on a surface opposite the element. Preferably, the position sensors are arranged at different distances from the rotating axis of the movable element.
The method subject to the invention for determining the frequency and/or amplitude of a vibrating structure comprises the steps: Determining a threshold value in the positive and/or negative half-wave of a vibration, determining the duration for exceeding and/or falling short of the threshold value during a vibration, and determining the frequency and/or amplitude from the duration of exceeding and/or falling short of the threshold value. In this manner, the vibration amplitude can be determined regardless of the distance between the vibrating element and a fixed element and the frequency of the vibration can be determined as well.
Advantageously, a threshold is determined for both the positive and the negative half-wave of the vibration and the frequency and/or the amplitude is determined from the duration of exceeding and/or falling short of the two threshold values. The threshold values in the positive and the negative half-waves may have the same value. Especially, they can be formed by mechanical steps.